Wire bonding apparatus

ABSTRACT

A wire bonding apparatus for semiconductor devices have a supporting block for holding a workpiece to which wires are bonded, a drive for driving the supporting block for holding the workpiece, the drive being capable of vertically moving and stopping the supporting block at a predetermined position, an optical means disposed above the supporting block and used for recognizing the pattern of the workpiece held on the supporting block, a storage means for prestoring each level of the workpiece so that the positions of not less than two places intended for recognition on the workpiece having difference in level conforms to the focal point of the optical means, and a control unit for controlling the drive for driving the supporting block, moving the workpiece to the position stored in the storage means and stopping the workpiece at that position. The supporting block is supported by a stroke bearing comprising a fixed part and a moving part which is moved and guided in the direction perpendicular to the fixed part. The drive for driving the supporting block comprises a cam shaft, a drive cam secured onto the cam shaft and generating a uniform velocity cam curve, a cam follower for coming contact with the drive cam, the cam follower being provided on the supporting block side, and a cam-driving pulse motor capable of rotating the cam shaft and electrically controlling the angle of rotation of the drive cam.

This application is a divisional of application Ser. No. 07/899,138filed Jun. 15, 1992, now U.S. Pat. No. 5,356,065; which is acontinuation of application Ser. No. 07/567,609 filed Aug. 15, 1990 nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wire bonding apparatus forsemiconductor devices and more particularly to a wire bonding apparatusfit for use in manufacturing high-density semiconductor devices withaccuracy.

2. Prior Art

Wire bonding apparatus and the like are increasingly desired to beprecise in dealing with circuit patterns which tend to becomeminiaturized and densified and also with high-density packaging. It isnow deemed essential for such wire bonding apparatus to correspond withthe miniaturization of not only bonding pads but also bonding leads onsemiconductor chips, for instance.

When leadframes, particularly thin leadframes of multi-pin LSI asintended workpieces, are handled, the well-known practice is torecognize patterns by means of an ITV camera for detecting what has beenmagnified through an optical lens barrel.

The wire bonding process stated above will be described with referenceto FIG. 4.

While mounting a semiconductor chip 2 on the surface, a leadframe 1 asthe workpiece, having been taken out of a supply magazine (not shown) isconveyed up to a locking position for bonding with both its lateralend-undersurfaces supported by a pair of guide rails 3a, 3b.

A switching pulse motor 4, a flighted screw 5 with left and rightportions threaded in opposite directions and the like are utilized forleveling and switching the pair of guide rails 3a, 3b, the guide rails3a, 3b being contracted at the locking position to laterally positionthe leadframe by abutting against it.

A block 7 for supporting a workpiece such as a heater block which ismade vertically freely moveable by a stroke bearing 6 is arranged underthe locking position, whereas a presser member 9 for holding down theleadframe is arranged above the locking position, the presser member 9being also made vertically freely moveable by a stroke bearing 8. Theleadframe 1 is vertically sandwiched between the block 7 for supportingthe workpiece and the presser member 9 and fixed at the lockingposition.

On the other hand, an optical lens barrel 11 for magnifying and guidingan image to a monitoring (CCD) camera 10 is disposed above the lockingposition of the leadframe 1. The monitoring camera 10 and the opticallens barrel 11 are secured to a bonding head 13 mounted on an X-Y table12. A bonding arm 15, fitted with a bonding tool 14 at its leading endis fitted to the bonding head 13 in such a way that it is capable ofrocking.

The pattern enlarged through the optical lens barrel 11 is detected bythe monitoring camera 10. Then the pattern on a combination of a bondingpad and an inner lead 1a of the leadframe 1 on the semiconductor chip 2is recognized and the shifting of the bonding position is corrected,whereby wires 16 may be bonded by means of a bonding tool 14 accurately.

In a wire bonding apparatus of the sort stated above, however, theshifting of the optical focal point due to the difference in levelbetween the surface of the semiconductor chip 2 and that of the innerlead 1a of the leadframe 1 may deteriorate the accuracy. As a result, ithas generally been arranged that the optical lens barrel 11 is madevertically movable via a rolling bearing to prevent the focal point fromshifting.

Moreover, there has been proposed a wire bonding apparatus for hybridICs in Japanese Patent Laid-Open No. 125639/1987. FIG. 5 shows the wirebonding apparatus disclosed therein.

A substrate fixing part 7' for holding a substrate 1 as an intendedworkpiece is made vertically movable via a motor 17 and a flighted screw18 extended perpendicularly and rotated as the motor 17 rotates, so thatthe substrate 1 is vertically moved in conformity with the focal pointof a monitoring camera 10.

In the case of the conventional wire bonding apparatus shown in FIG. 4,however, the problem is that the shifting of the optical focal point dueto the difference in level between the surface of the semiconductor chipand that of the inner lead results in reduced accuracy. If the leadframeis to be vertically moved to solve the aforementioned problem, the blockfor supporting the workpiece together with the leadframe presser willhave to be vertically moved. Even if the block for supporting theworkpiece together with the leadframe presser is made verticallymovable, the guide rails may rub against the edge faces of theleadframe, thus subjecting the inner lead of the leadframe and thebonded wire to oscillation, which may damage both of them and hencedevelop structural defects.

With the optical lens barrel arranged vertically movable, moreover, theprovision of a mechanism for vertically moving the lens barrel will notonly induce an increase in weight and cost but also create factors inreducing accuracy attributable to clearance and backlash in the slideportion of the optical lens barrel.

More specifically, owing to the general mechanical accuracy required torender the mechanism vertically movable, the problem of positionalreproducibility of the vertical movement and that of the horizontalpositional reproducibility derived from its vertical movement maysacrifice the accuracy. If it is attempted to vertically move theoptical lens barrel by approximately 0.5 mm which is equivalent to thethickness of the semiconductor chip, for instance, the optical lensbarrel may slightly rock as it vertically moves. Since the horizontalchange resulting from the rocking motion is infinitesimal, it becomeshardly correctable.

In other words, a gap of several microns with the rolling bearing, forinstance, constitutes an important factor in making inaccurate thebonding position at the stage where the accuracy of the bonding positionof the order of microns is narrowed down.

In the case of the conventional wire bonding apparatus shown in FIG. 5,the substrate as the intended workpiece is vertically moved inconformity with the focal point of the optical system in order to adjustthe substrate to the reference level of the bonding tool as bondinglevel after the pattern has been recognized. For this reason, thebonding thus effected is considered inclusive of errors ascribed to thevertical movement of the substrate.

Wire bonding by means of leadframes is not at a problematical level aslong as the amount by which the perpendicularity of a bonding tool isaffected because of the difference in level resulting from thedifference in the thickness of the leadframe, the level of asemiconductor chip and the like is within the range of approximately tanΘ=0.7 mm/1.10 mm, Θ=0.4°. Consequently, if the perpendicularity of thebonding tool is within the above range of errors, the merit of takingthe perpendicularity into consideration may be low; on the contrary, thedemerit is considered rather high.

Since the flighted screw is employed for driving the substrate to movevertically, it is necessary to vertically move the leadframe presser forfastening the leadframe to the block for supporting the workpiece inaddition to the member for fixing the substrate when, for instance, theleadframe is handled. As a result, two drives have to be installedaround the member for fixing the substrate, causing an increase ininstallation space and cost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a wire bondingapparatus which is relatively simple and compact in construction andcapable of controlling the vertical movement of a workpiece inconformity with the focal point of an optical system for recognizing apattern horizontally moving at high speed without increasing the weightof the optical system, of materializing marked bonding accuracy byeliminating the cause of shifting in connection with the verticalmovement of the workpiece, and of switching one to another kind ofworkpiece in a short time without causing damage to the workpiece.

In order to accomplish the foregoing object, the wire bonding apparatusfor semiconductor devices according to the present invention comprises asupporting block for holding a workpiece conveyed by a conveyor, thesupporting block being capable of vertical movement and of stopping atany given position; an optical means arranged above the workpiece andused for recognizing the pattern of the workpiece held by the supportingblock; and a storage means for prestoring not less than two positions tobe recognized of the workpiece, these positions conforming to the focalpoint of the optical means but differing in level, wherein after patternrecognition is made at one of the positions with respect to less thantwo recognizing levels on the same workpiece, the level of the workpieceis shifted to another where pattern recognition is effected at thatposition, whereby the required correction of the shifted position isimplemented. It may also be made possible to arrange a drive camoperating with a uniform velocity curve in such a way that, by rotatingthe cam drive, the supporting block can be moved vertically and stoppedat any position via a motor capable of electrically controlling theangle of rotation of the cam, and to recognize the pattern comparativelyliberal in specification first among those different in level.

With the arrangement stated above, highly accurate wire bonding can beimplemented according to the present invention through the steps of, forinstance, moving the workpiece to adjust the focal point of the opticalsystem to what is liberal in the specification required among not lessthan two levels to be recognized of the intended workpiece, recognizingthe pattern at that position, moving the workpiece further to adjust thefocal point of the optical system to another level to be recognized,recognizing the pattern at that position, and then correcting theshifting of wiring to be bonded by means of the pattern recognition.

As the present invention is thus arranged, highly accurate wire bondingcan be accomplished by vertically moving the workpiece in conformitywith the focal point of the optical system and besides eliminating theprime factor in the shifting relating to the vertical movement of theworkpiece. Moreover, it is possible to not only switch one to anotherkind of workpiece in a short time but also deal with a workpiecemounting a semiconductor chip having difference in level.

In addition, the wire bonding apparatus provided according to thepresent invention can be made relatively simple and compact inconstruction without impairing high-speed wire bonding functions andincreasing the weight of the optical system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view illustrating a wire bonding apparatusembodying the present invention.

FIG. 2 is a timing chart illustrating the timing of conveyance,operations of guide rails, a switching pulse motor and a cam drivingpulse motor, and pattern recognizing and wire bonding actions.

FIG. 3 is a graph illustrating curves drawn by a cam for verticallymoving a supporting block and another for holding down a leadframe.

FIG. 4 is an elevational view illustrating a conventional wire bondingapparatus.

FIG. 5 is an elevational view illustrating another conventional wirebonding apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will subsequently given of an embodiment of the presentinvention with reference to FIGS. 1 to 3, inclusive.

While mounting a semiconductor chip 2 on the surface, a leadframe as theworkpiece, having been taken out of a supply magazine (not shown) isconveyed up to a locking position for bonding with both its lateralend-undersurfaces supported by a pair of guide rails 3a, 3b.

As in the case of the conventional wire bonding apparatus shown in FIG.4, a switching pulse motor 4, a flighted screw 5 (not shown in FIG. 1with left and right portions threaded in opposite directions and thelike are utilized for leveling and switching the pair of guide rails 3a,3b, the guide rails 3a, 3b being contracted at the locking position tolaterally position the leadframe by abutting against it.

A supporting block 7 for holding the workpiece such as a heater blockwhich is vertically freely moveable by a stroke bearing 6, is arrangedunder the locking position. A tension spring 19 is stretched between thefixed and the moving part 6a, 6b of the stroke bearing 6. With thistensile force, a cam follower 20, secured to the moving part 6b, iscaused to abut against a cam 21 for vertically moving the block 7.

Moreover, a presser member 9, vertically freely moveable by a strokebearing 8, is arranged above the locking position. A tension spring 22is stretched likewise between the fixed and the moving part 8a, 8b ofthe stroke bearing 8. With this tensile force, a cam follower 23 securedto the moving part 8b is caused to abut against a cam 24 for verticallymoving a leadframe presser.

These two cams 21, 24 are secured to the same cam shaft 26, rotatablysupported by a bracket 25, and the cam shaft 26 is arranged so that itis rotated as a cam-driving pulse motor 27 rotates.

As shown in FIG. 3, the cam 21 for vertically moving the supportingblock is so shaped as to cause the cam follower 20 to ascend in a sinecurve within the range of angles of rotation a-b during the forwardrotation, to stop within the range of angles of rotation b-c, to ascendin a uniform velocity curve within the range of angles of rotation c-d,to stop within the range of angles of rotation d-e, and to descend in asince curve within the range of angles of rotation e-f (this order isreversed during the backward rotation).

The cam 24 for vertically moving the supporting block is so shaped as tocause the cam follower 23 to descend within the range of angles ofrotation g-h during the forward rotation, to stop within the range ofangles of rotation h-i, to descend in a sine curve within the range ofangles of rotation i-j, to stop within the range of angles of rotationj-k, and to ascend in a since curve within the range of angles ofrotation k-1 (this order is reversed during the backward rotation).

When the cam shaft 26 is rotated forward, the supporting block 7 thusascends via the cam follower 20, whereas the leadframe presser 9descends via the cam follower 23. When the leadframe 1 ascends by apredetermined amount, both its lateral end portions are verticallysandwiched and held between the supporting block 7 and the leadframepresser 9 and caused to ascend further. The cam 24 for verticallyholding down the leadframe is detached from the cam follower 23 and theleadframe 1 is held by the elastic force of the tension spring 22therebetween. While the leadframe 1 is thus held by the elastic force ofthe tension spring 22 therebetween, the supporting block 7 and hence theleadframe 1 are caused to ascend and descend along the uniform velocitycurve of the cam 21 for vertically moving the supporting block.

By controlling the lifting amount of the leadframe 1 along the uniformvelocity curve of the cam 21 for vertically moving the supporting block,i.e., the angle of rotation of the cam shaft 26, moreover, the leadframe1 can be stopped in position. The angle of rotation of the cam-drivingpulse motor 27 is controlled via a driver 28 according to an outputsignal from a CPU 29. The CPU 29 is fitted with a memory device 30,which is stored with stop positions of the leadframe 1 and thesemiconductor chip 2 (i.e., a predetermined angle of rotation of the camshaft 26 for vertically moving the supporting block), the stop positionsconforming to the focal point of an optical lens barrel 11. The CPU 29is also fitted with a condition input device 31.

The optical lens barrel 11 for enlarging and guiding an image to amonitoring (CCD) camera 10 is disposed at the position offset in the X-Ydirections with respect to a bonding tool 14 above the locking positionof the leadframe 1. The monitoring camera 10 and the optical lens barrel11 are secured to a bonding head 13 mounted on an X-Y table 12 whosemovement in the X-Y directions is controllable.

A bonding arm 15 fitted with the bonding tool 14 at its leading end isfitted to the bonding head 13 in such a way that it is capable ofvertically rocking round a rocking fulcrum 15a, the bonding tool 14having a bonding wire 16 passed therethrough.

The operating timing of the embodiment stated above will subsequently bedescribed with reference to FIG. 2.

The leadframe 1 is first conveyed along the guide rails 3a, 3b andproperly positioned in the direction of conveyance before being stopped.In this state, the pulse motor 4 for switching the guide rails isrotated forward by the predetermined amount that has been prestored (seeFIG. 3) and the guide rails 3a, 3b are contracted, so that the leadframe1 is laterally positioned (timing A).

Then the cam shaft 26 is rotated forward by the predetermined angle ofrotation Θ₁ shown in FIG. 3 via the cam-driving pulse motor 27 to ascendthe supporting block 7 and simultaneously descend the leadframe presser9. Both the lateral surface and undersurface of the leadframe 1 are heldbetween the supporting block 7 and the leadframe presser 9 (timing B).The cam 24 for vertically moving the leadframe presser and the camfollower 23 are separated from each other and the tensile force of thetension spring 22 is used to hold the leadframe 1 therebetween.

The pulse motor 4 for switching the guide rails is rotated backward toopen the guide rails 3a, 3b in order to provide gaps between the guiderails 3a, 3b and the respective side edge faces of the leadframe 1.

While the guide rails 3a, 3b are kept open, the supporting block 7 andthe leadframe 1 are made to ascend up to a position where the lead faceof the inner lead 1a conforms to the focal point of the optical lensbarrel 11.

More specifically, the cam-driving pulse motor 27 is rotated forward bythe predetermined angle of rotation Θ₂ ; shown in FIG. 3 via the camshaft 26 to ascend the supporting block 7 via the cam follower 20 alongthe uniform velocity curve of the cam 21 for vertically moving thesupporting block. The angle of rotation Θ₂ is what has been prestored inthe memory device 30 as the first stop position of the supporting block7.

In this state, the pattern of the inner lead 1a of the leadframe 1 isrecognized through image processing in the monitoring camera 10 (timingC).

Subsequently, the supporting block 7 and hence the leadframe 1 arecaused to descend by the amount equivalent to the difference in levelbetween the inner lead 1a and the semiconductor chip 2, i.e., up to theposition where the focal point of the optical lens barrel 11 conforms tothe bonding pad of the semiconductor chip 2. In other words, by rotatingthe cam shaft 26 backward by the predetermined angle of rotation Θ₃ ;shown in FIG. 3 via the cam-driving pulse motor 27, the supporting block7 is caused to descend via the cam follower 20 along the uniformvelocity curve of the cam 21 for vertically moving the supporting block.The angle of rotation Θ₃ is what has been prestored in the memory device30 as the second stop position of the supporting block 7.

In this state, the pattern of the bonding pad of the semiconductor chip2 is recognized through image processing in the monitoring camera 10(timing D).

The shifting of the initially set coordinates of the leadframe 1 and thesemiconductor chip 2 obtained from the pattern recognition stated aboveis utilized for correcting the respective positions of the wire bondingcoordinates. While the supporting block 7 is stopped at the second stopposition, wire bonding is effected by means of the bonding tool 14.

After the termination of the wire bonding operation, the cam shaft 26 isrotated backward by Θ₄, shown in FIG. 3 via the cam-driving pulse motor27 to return both the cams 21, 24 to the starting point. The supportingblock 7 is consequently caused to descend through both the cam followers20, 23, whereas the leadframe presser 9 is caused to ascend. Theleadframe 1 is thus set free. Although the leadframe 1 is caused todescend while supported by the supporting block 7 at this time, it isdetached from the supporting block 7 simultaneously with its arrival atthe guide rails 3a, 3b, both the lateral end portions being supported bythe guide rails 3a, 3b thereafter (timing E).

The leadframe 1 that has been subjected to wire bonding is then conveyedby the guide rails 3a, 3b and accommodated in a receiving magazine. Aseries of bonding operations is thus completed.

The present invention is needless to say not limited to the sequence andtiming in the embodiment shown nor restricted by the elements such aspulse motors, a CCD camera and the like.

The reason for making the pattern recognition of the inner lead 1afollowed by that of the semiconductor chip 2 is attributed to the factthat higher accuracy is required because the bonding pad of thesemiconductor chip 2 is normally smaller than the inner lead 1a. Inother words, the pattern of what is liberal in specification is firstrecognized to increase accuracy and therefore it is dependent on thebonding accuracy required which one of the patterns is to be recognizedfirst.

Moreover, the level of the supporting block to be stopped is determinedby the thickness of the leadframe 1 and the semiconductor chip 2, andthe relative distance between the faces of the inner lead and thesemiconductor chip.

With respect to a workpiece with a kind of leadframe different in levelor a semiconductor chip different in thickness being mounted on the sameleadframe, it can be dealt with by prestoring the level of thesupporting block to be stopped in the form of numerical values and thelike in the CPU for controlling the angle of rotation of the cam-drivingpulse motor beforehand.

What is claimed is:
 1. A method of accurately positioning a wire bondingapparatus for executing a bonding operation with said wire bondingapparatus, comprising the steps of;a) conveying to a predeterminedposition a workpiece to which wires are to be bonded while opposite endsof said workpiece as seen in the lateral direction are supported by apair of guide rails, b) holding said workpiece at said predeterminedposition not only from above but also from below by a leadframe presserdisposed above said workpiece as well as a leadframe supporting blockdisposed below said workpiece, c) positionally coinciding a firstvisually recognizable position on said workpiece with a focal point ofoptical means disposed above said predetermined position, d)positionally coinciding a second visually recognizable position on saidworkpiece with said focal point of said optical means, e) making apositional correction for a position to be assumed by said workpiecebased on said first and second virtually recognizable positions on saidworkpiece; and f) thereafter, executing said bonding operation,whereineach of the steps of positionally coinciding said first and secondvisually recognizable positions on said workpiece with said focal pointof said workpiece is practiced by raising and lowering said workpiecesupporting block disposed below said workpiece, and said leadframepresser is placed on said workpiece supporting block without any forceapplied thereto, whereby said leadframe presser is displaced inconformity with a displacement of said leadframe supporting block. 2.The method according to claim 1, wherein said workpiece supporting blockand said leadframe presser are raised and lowered by two drive camswhich are disposed on a common cam shaft in a spaced relationship. 3.The method according to claim 1, wherein each of the steps ofpositionally coinciding said first and second visually recognizablepositions on said workpiece with said focal point of said optical meansfurther includes:a step of displacing said workpiece supporting block inthe upward direction by rotating a cam shaft by a first predeterminedrotational angle so as to allow said first visually recognizableposition on said workpiece to positionally coincide with said focalpoint of said optical means, a step of visually recognizing a pattern ofsaid workpiece at said first visually recognizable position on saidworkpiece by actuating said optical means, a step of displacing saidworkpiece supporting block in the downward direction by rotating saidcam shaft by a second predetermined rotational angle so as to allow saidsecond visually recognizable position on said workpiece to positionallycoincide with said focal point of said optical means, and a step ofvisually recognizing a pattern of said workpiece at said second visuallyrecognizable position on said workpiece by actuating said optical means.4. The method according to claim 3, wherein said first and secondpredetermined rotational angles corresponding to said first and secondvisually recognizable positions on said workpiece are preliminarilystored in storing means.
 5. The method according to claim 1, whereinvisual recognition of said first and second visually recognizablepositions on said workpiece is conducted by first visually recognizingone of said first and second visually recognizable positions on saidworkpiece having a requirement for a low accuracy.
 6. The methodaccording to claim 1, wherein the step of executing a wire bondingoperation is practiced at said second visually recognizable position onsaid workpiece by actuating a bonding tool after said second visuallyrecognizable position on said workpiece positionally coincides with saidfocal point of said optical means.